The present invention relates generally to a zoom lens system and an imaging system that incorporates it, and more particularly to a zoom lens system that has a zoom ratio of at least 4 fit for en electronic image pickup device such as a CCD and an imaging system that incorporates it.
In recent years, digital cameras have gained popularity in some categories from a business-application, multifunctional type to a portable low-end type. In view of the portable low-end type category in particular, a primary object of the invention is to provide a technology capable of achieving a video or digital camera that has a relatively high zoom ratio yet is more slimmed down with high image quality.
The chief bottleneck to reducing the depth dimension of cameras is the thickness of the surface nearest to the object side of an optical system, especially a zoom lens system, to an image pickup plane. Recently, the use of a so-called collapsible lens mount, wherein an optical system is unfolded from within a camera body at the time of taking and folded down into the camera body during carrying-along, has become the main current of the camera industry.
An image pickup device of reduced size will suffice for compactness and slimming-down. With the same number of pixels, however, it is required to make pixel pitch small; poor sensitivity must be compensated for by an optical system. The same also holds for the influence of diffraction. In other words, there is the need of an optical system having a decreased F-number. Among such optical systems known so far in the art, there is an optical system comprising, in order from its object side, a first lens group having positive refracting power, a second lens group having negative refracting power, a third lens group having positive refracting power and a fourth lens group having positive refracting power, four lens groups in all, wherein upon zooming the space between adjacent lenses varies. For instance, patent publication 1, 2, 3, 4, and 5 discloses a zoom lens system that is composed of a relatively small number of lenses with a zoom ratio of about 4 to 5, and so is fit for use with an electronic imaging system; however, there are some problems such as unsatisfactory compactness due to an increased total length. Patent publication 6, 7, 8, and 9 discloses a zoom lens system having a zoom ratio of about 3, and patent publication 10 discloses an optical system having a zoom ratio of about 3 and fit for use with silver-halide films. A problem with these optical systems is that the F-number is of as large as about 3.5 at the wide-angle end.
Patent Publication 1
JP(A) 2003-315676
Patent Publication 2
JP(A) 2003-43357
Patent Publication 3
JP(A) 2001-42215
Patent Publication 4
JP(A) 2004-12639
Patent Publication 5
JP(A) 2004-12638
Patent Publication 6
JP(A) 2001-356269
Patent Publication 7
JP(A) 2001-242379
Patent Publication 8
JP(A) 2001-194586
Patent Publication 9
JP(A) 2000-1881709
Patent Publication 10
JP(A) 57-5012